1. Field of the Invention
This invention relates to heat-sensitive compositions and thermographic sheet material produced therefrom, and is particularly directed to compositions useful for producing thermally sensitive papers and films for use with thermal printing devices.
2. Description of the Prior Art
For many years, heat-sensitive imaging sheets have been used for copying papers, thermal print papers, recording papers, and labeling papers. Futaki, et al, U.S. Pat. No. 3,829,401 discloses a heat sensitive recording composition which comprises
(1) a colorless or light-colored color forming compound selected from the group consisting of leuco lactone and spiropyran compounds, PA0 (2) a phenol compound capable of causing color formation of said color forming compound upon heating, and PA0 (3) a three-dimensionally cross-linked phenol resin which is a condensation reaction product of at least one lower aliphatic aldehyde, lower aliphatic aldehyde producing agent or a lower alkyl vinyl ether and a phenol compound having at least 3 ortho positions or para positions or ortho and para positions to the phenolic hydroxyl group free of substituents. PA0 4-tert-butylphenol PA0 3-methyl-6-tert-butylphenol PA0 4-methyl-2-tert-butylphenol PA0 2-phenylphenol PA0 4-phenylphenol PA0 2,4-dimethyl-6-tert-butylphenol PA0 2,4-di-tert-butylphenol PA0 2,6-di-tert-butylphenol PA0 4-methyl-2,6-di-tert-butylphenol phenol PA0 4,4'-thiodiphenol PA0 4,4'-sulfonyldiphenol PA0 4,4'-isopropylidenediphenol (Bisphenol A) PA0 4,4'-thiobis(3-methyl-6-tert-butylphenol) PA0 p,p'-sec-butylidenediphenol PA0 2,2'-methylenebis(4-methyl-6-tert-butylphenol) PA0 4,4'-methylenebis(2,6-di-tert-butylphenol) PA0 1,3,5-trimethyl-2,4,6-tris(3,5-di-tert-butyl-4-hydroxybenzyl)benzene PA0 1,3,5-tris(4-tert-butyl-3-hydroxy-2,6-dimethyl benzyl)-1,3,5-triazine-2,4,6-(1H,3H,5H)-trione
Futaki, et al, U.S. Pat. No. 3,846,153 discloses a thermal recording sheet containing the phenol compound having the formula ##STR1## at least one colorless or light-colored lactone compound capable of reacting with said phenol compound upon heating to form a color, and a binder.
Truitt, U.S. Pat. No. 3,953,659 discloses a heat-sensitive print sheet comprising a thin flexible sheet material and including a heat-sensitive layer comprising:
a. a heat sensitive color producing formulation including a normally solid iron salt of a fatty acid and a diphenolic compound; PA1 b. a binder comprising cellulose acetate; PA1 c. acetone as a solvent for the binder; and PA1 d. water as a non-solvent blush material for the binder. PA1 (1) Monophenols PA1 (2) Bisphenols PA1 (3) Polyphenols containing more than two phenolic groups PA1 R.sub.2, R.sub.3, and R.sub.4 are independently hydrogen, --OH, an aryl radical, or an alkyl radical of 1 to 6 carbon atoms, provided that R.sub.3 cannot be --OH if either R.sub.2 or R.sub.4 is --OH, or if both R.sub.2 and R.sub.4 are --OH.
One type of heat-sensitive recording paper in wide use is generally referred to as a ferric-phenolic system. Such a recording paper generally comprises a paper sheet bearing a layer containing (1) a ferric salt of an organic acid, and (2) a phenol which reacts with the ferric salt to form a visible image when the paper is heated. Miller, et al, U.S. Pat. No. 2,663,654 describes heat-sensitive systems of the ferric-phenolic type.
Because of the improvement in thermal printing devices, a need has arisen to provide heat-sensitive recording sheets of higher speed and better quality. By higher speed, it is meant that the image appears more rapidly upon application of heat. By better quality, it is meant that the image exhibits greater stability and better resolution. Dye-based heat-sensitive imaging sheets, such as those described in U.S. Pat. Nos. 3,829,401 and 3,846,153, exhibit rapid thermal response, good resolution, and good contrast. However, the images produced tend to fade when exposed to ultraviolet radiation, and the imaging sheets are very susceptible to chemical attack. Contact with hand lotion, grease, alcohol, or adhesive on transparent tapes can readily obliterate the image. Accordingly, it is still desirable to employ a conventional ferric-phenolic system for heat-sensitive recording sheets because this system exhibits exceptional permanence, i.e. good stability, and good resistance to most common chemicals and ultraviolet radiation. In addition, ferric-phenolic thermal imaging systems are capable of providing black images at low cost. Although dye-based thermal imaging systems are capable of providing black images, the cost of a conventional dye-based system is quite often significantly higher than the cost of a conventional ferric-phenolic system.